#div1{
    position: relative;
    left: 50%;
    width: 1000px;
    height: 600px;
    margin-left: -500px;
    border: 1px solid;
    background-color: silver;
}
#div2{
    position: absolute;
    top:40%;
    left: 50%;
    width: 600px;
    height: 300px;
    margin-left: -150px;
    border: 1px solid;
    background-color: red;
}